The first step in this sequence is the combustion of carbon $C$ to produce carbon dioxide $CO2$.

C + O2 -> CO2

The next step is the reaction of carbon dioxide $CO2$ with calcium oxide $CaO$ to form calcium carbonate $CaCO3$.

CO2 + CaO -> CaCO3

The third step involves the decomposition of calcium carbonate $CaCO3$ to produce carbon dioxide $CO2$ and calcium oxide $CaO$.

CaCO3 -> CO2 + CaO

The final step is the reaction of carbon dioxide $CO2$ with sodium oxide $Na2O$ to form sodium carbonate $Na2CO3$.

CO2 + Na2O -> Na2CO3

Overall, the complete sequence of reactions is:

C + O2 -> CO2
CO2 + CaO -> CaCO3
CaCO3 -> CO2 + CaO
CO2 + Na2O -> Na2CO3